Local Current Variation by Depth inGeobacter SulfurreducensBiofilms

New methods of detecting local extracellular electron transfer rates in electrochemically active biofilms (EABs) are needed to quantify the role of microscale gradients in extracellular electron transfer. We utilized electrode-respiring Geobacter sulfurreducens biofilms as a model EAB to demonstrate local current measurement by depth using microelectrodes with a graphite tip covered by biofilm. We grew G. sulfurreducens biofilms on an electrode (electrode-biofilm) and on the tip of a microelectrode (microelectrode-biofilm) and monitored current generation. The microelectrode-biofilm current was negatively affected by the current produced in the electrode-biofilm; this resulted in a change in current, Δ. We found a 30% decrease in current from the top of the electrode-biofilm to the bottom of the electrode-biofilm. We attribute the current loss at the microelectrode-biofilm to exposure to local microscale conditions that reduced the ability of the microelectrode-biofilm to participate in extracellular electron transfer. It is possible that such microscale gradients as pH, redox and acetate caused a 30% current drop between top and bottom. The new technique developed in this research provides a measure of the local impact of extracellular electron transfer occurring within a thick biofilm.